1. Field of the Invention
The present invention relates to a development device which forms an image by an electrophotographic system, an electrostatic recording system, and the like. Particularly the invention relates to an image forming apparatus which prevents generation of trouble with an information defect, caused by a partially missing specific pattern, such as machine-readable information typified by a barcode and a QR code (registered trademark). The development device generates the partially missing specific pattern.
2. Description of the Related Art
The electrophotographic system is one of well-known printing systems used in a copying machine and a printer. Recently, attention focuses on POD (Print On Demand), and high-speed printing capability and photographic image printing are demanded for the electrophotographic system. Therefore, high-quality and fine print image is also demanded.
Usually, a one-component developer mainly containing non-magnetic toner or magnetic toner or a two-component developer mainly containing the nonmagnetic toner and the magnetic carrier is used in the development device included in the electrophotographic system or electrostatic recording system image forming apparatus. Particularly, in the color image forming apparatus in which a full-color or multi-color image is formed by the electrophotographic system, from the viewpoint of image color, the two-component developer is used in the most development devices. Using the development devices, toner images are superimposed on a sheet or an intermediate transfer member, and the toner is melted and fixed to the sheet to form the color image.
On the other hand, recently documents on which confidential information is listed are strictly managed for the purpose of information security. In the field of image formation, when the document on which confidential information is listed is formed, a mark indicating a confidential document is added to the document on which confidential information is listed in order to clearly recognize that the confidential information is listed on the document. In order not to produce a duplicate, the image forming apparatus interrupts or disables the image formation when the image forming apparatus recognizes the document as confidential document including the confidential information. In order to operate the image forming apparatus in the above-described manner, the added mark or information is formed by easily recognizable mark or so-called machine-readable information. The mark is expressed by characters such as confidence and secret. Examples of the machine-readable information include an encrypted pattern, the barcode, and the QR code.
In the two-component development device, usually a non-image part has a potential difference opposite an image part such that the toner is not developed in the non-image part (that is, such that fog is not generated) (hereinafter referred to as “fog removal potential”). Because the toner has a fixed polarity in the development device, the toner is kept away from the non-image part by the fog removal potential.
In the two-component development device, usually a rotating speed of a developer bearing member is enhanced one to two times a rotating speed of a surface of an image bearing member such that the toner is effectively supplied to the image bearing member.
A toner charging amount depends on temperature and humidity of an environment in which the toner is used or a mixture ratio to a carrier. Usually the toner charging amount is lowered in the high-temperature and high-humidity environment, and the toner charging amount is increased in the low-humidity environment.
However, in forming the barcode or QR code image which is of the machine-readable information, sometimes the image defect causes the development device to generate the missing information.
When the partially missing machine-readable information is generated, there is a possibility that the information cannot be reproduced by the machine reading, and the missing information is made worse. Frequently the machine-readable information typified by the barcode and the QR code is used as a security code. However, when the image defect is generated, the machine-readable information does not function as the security code. Even in the QR code used to prevent duplication, the QR code is not recognized as the machine-readable information due to the image defect, which possibly generates a problem in that duplication is enabled.
Therefore, for example, Japanese Patent Application Laid-Open No. 2000-206794 discloses a device which reduces a toner scrabble phenomenon in order to suppress the generation of the image defect according to a developer concentration and a beam diameter.
The information missing phenomenon caused by the image defect is remarkably generated in the case of the low toner charging amount. In the case of the low toner charging amount, because an electrostatic adsorption force with the image bearing member is decreased, the toner is easily separated from the image bearing member, which causes the image defect.
Furthermore, the information missing phenomenon is remarkably generated in the case of the thin (latent image having a small potential change) electrostatic image (hereinafter referred to as “electrostatic latent image” or simply referred to as “latent image”) such as a narrow line and half tone. Usually laser power is controlled such that the toner amount developed to the latent image generated by an integrated light quantity in forming the solid latent image becomes a predetermined density. However, in reproducing the narrow line, sometimes the integrated light quantity becomes small for the solid latent image, and the potential changes of the narrow-line latent image part and other parts are small in comparison with the potential change of the solid latent image.
In the case where a later-mentioned pulse width modulation circuit is used, it is necessary to shorten a pulse width in order to reproduce a line width of the narrow line. Therefore, even if the laser is driven with the pulse width for reproducing the narrow line, the laser power rises insufficiently, and the narrow-line latent image is formed with insufficient laser power, whereby the latent image becomes thin. In the half-tone latent image, the latent image tends to become thin due to the similar reason. Accordingly, the image defect is easily generated because the electrostatic adsorption force between the toner and the image bearing member is weakened for the thin latent image.
The phenomenon will specifically be described with reference to the drawings. FIG. 11 illustrates the state in which the image defect is generated. A part of a developer bearing member (development sleeve) SL is illustrated on the left side of FIG. 11, a part of an image bearing member (photosensitive drum) Dr is illustrated on the right side of FIG. 11, and the carrier and toner of the two-component developer are illustrated in an area between the development sleeve SL and the photosensitive drum Dr. A drum surface potential (Vd) at a solid white part, a drum surface potential (V1) at a solid part, and magnitude of a development potential (Vdc) applied to the development sleeve SL are expressed in (r, φ) axis while correlated with a position of the drum surface.
Referring to FIG. 11, in the solid white part, for the drum surface potential surrounded by a broke line, a force is applied to the toner by a potential difference with the development potential Vdc such that the toner is pressed toward a development sleeve direction. The drum surface potential acts as a so-called fog removal potential (Vback) which keeps the toner away from the solid white part. On the contrary, a force is applied to the carrier having the polarity opposite to the toner such that the carrier is pressed toward the photosensitive drum direction.
Accordingly, in the part surrounded by the broken line, because the toner is pressed against the development sleeve SL by the fog removal potential, the amount is increased near the development sleeve, and the carrier amount is increased near the photosensitive drum while the toner amount is decreased. In the narrow-line latent image part, because the toner is developed for the photosensitive drum, the toner amount is schematically increased only in the narrow-line part.
Near the photosensitive drum of the part surrounded by the broken line, the many carriers whose surfaces are exposed exist because the toner amount is decreased. Accordingly, because the carrier whose surface is exposed generates a charge having the polarity opposite to the toner (so-called countercharge), the carrier easily generates an effect (scavenging phenomenon by countercharge) of peeling off the toner developed on the photosensitive drum. Particularly, in the case where a high-resistance carrier is used, the scavenging phenomenon tends to become prominent because a charge attenuation time is required.
In order to efficiently supply the toner from the development sleeve, the rotating speed of the development sleeve is set 1.5 times the rotating speed of the photosensitive drum. In the case of FIG. 11, the carriers in the countercharge state perform the scavenging to the narrow line on the photosensitive drum to peel off a part of the toner image. The problem is described only by way of example because the scavenging becomes prominent when a circumferential speed of the development sleeve is higher than a circumferential speed of the photosensitive drum. Additionally, the similar problem is possibly generated even if the circumferential speed of the development sleeve is equal to the circumferential speed of the photosensitive drum. That is, in the non-image part adjacent to the toner image, the toner image is possibly disturbed by receiving the electric force from the carrier.
FIG. 12 schematically illustrates the state in which the scavenging is generated in the narrow-line toner image. FIG. 12A illustrates the ideal toner image state in which the scavenging is not generated. FIG. 12B illustrates the state in which a part on the upstream side in the rotating direction (latent image conveying direction) of the photosensitive drum, that is, a rear end part of the narrow line is scrabbled by the scavenging (hereinafter referred to as “rear-end scrabble”).
Not only the rear-end scrabble phenomenon becomes a trouble in visualizing the latent image, but also the rear-end scrabble phenomenon leads to the information missing in reproducing the machine-readable information such as the barcode. FIGS. 13A to 13D schematically illustrate the states of the machine-readable information when the rear-end scrabble is generated. FIG. 13A illustrates the barcode, FIG. 13C illustrates the QR code, and FIGS. 13B and 13D illustrate the states in which the rear-end scrabble partially scrabbles the machine-readable information to generate the information missing.
In recording the machine-readable information, it is necessary to prevent the generation of the rear-end scrabble phenomenon. Particularly, a configuration in which the rear-end scrabble phenomenon is effectively suppressed is demanded when a Tribo-charge is lowered in the toner.